The present invention generally relates to a method and apparatus for the acoustic inspection of a borehole fitted with a casing and passing through ground formations. More particularly, the invention relates to a method and apparatus for providing environmental correction to measurements obtained by using an acoustic pulse technique for forming signals representative respectively of one or more data items relating to the casing and to the material which surrounds it. The data items relating to the casing and to the materials surrounding the casing include, in particular, the quality of the connection of the cement with the casing, the casing thickness, and the roughness of the inside wall of the casing. For purposes of definition herein, it should be understood that the "environmental correction" and the "environment" as used herein will refer to the correction for, and the environment of the investigating tool, i.e. the mud or fluid surrounding the tool in the casing, and the casing itself.
Generally, when a borehole has reached a desired depth, casing is placed therein and cement is injected into the annular space formed between the casing and the wall of the borehole in order to prevent any hydraulic communication between the various geological layers. To determine whether any such unwanted communication still exists, measurements are performed by means of a logging tool for determining the quality of the link between the cement and the casing. Logging tools using acoustic waves to perform such measurements have been available to the art for a very long time. However, most of the logging tools rely on an average circular measurement and/or an average longitudinal measurement (along the casing), and consequently are not capable of identifying spot phenomena, such as longitudinal hydraulic communication channels.
Of the various techniques seeking to employ vertical and radial resolution to cement-to-casing quality measurements, that described in U.S. Pat. No. 4,255,798 to Havira, and assigned to the assignee herein, appears to be of greatest interest. It generally comprises: sending an acoustic pulse at a radial sector of a casing, the pulse constituting acoustic waves whose frequencies are chosen so as to induce thickness resonance between the outer wall and the inner wall of the casing; determining the energy in a reverberation segment of the reflected signal; and forming a signal which is representative of the reflected signal to characterize the quality of the connection of the cement to the casing behind the said radial sector of the casing. the reverberation segment of the reflected signal is chosen so as to be substantially representative of the acoustic reverberation between the walls of the casing. Rapid damping of the resonance, i.e. low energy in the segment, indicates the presence of cement behind the casing, while slow damping, i.e. high energy, indicates the absence of cement.
A logging tool using the above-described technique is described in Schlumberger's commercial brochure entitled "Cement Evaluation Tool" which was published in June 1983. The sonde of this tool is centered in the casing. The tool generally comprises nine transmitter/receiver transducers. Eight transducers are helically distributed at 45 degrees so as to obtain good coverage of the periphery of the casing. Acoustic pulses are fired by the eight transducers sequentially and are likewise received sequentially before being analyzed and sent to the formation surface where they are processed. The ninth transducer, also referred to as a reference transducer, is pointed along the axis of the casing towards a reflecting wall which is planar and disposed at a fixed distance from the reference transducer. The reflected signal detected by the ninth transducer is used to determine the speed of propagation of the acoustic wave through the in situ fluid, as the distance between the reference transducer and reflecting wall is known, and the time interval between the emission and reception of the acoustic wave is determined. Using this speed of wave propagation, it is possible to determine the apparent radius of the casing for each of the eight transducers. This radius is a particularly interesting item of information, since it enables a detection of any deformation of the casing, and it also enables the centering of the sonde inside the casing to be monitored to give an indication of the validity and the quality of the recorded measurements.
The measurements performed with the above-described tool have confirmed that it is possible to identify certain hydraulic communication channels. An example of such an identification is to be found in FIG. 5 of French Pat. No. 2,491,123 to B. Seeman, assigned to the assignee herein, which describes a method and an apparatus for obtaining a recording of the quality of the cement. Despite this success, quantitative interpretation of such measurements have led to unexpected problems. In particular, there has been a divergence between measurements depending on the depth at which they are performed, the type of mud or fluid found in the borehole casing and on the geometry of the casing being analyzed.
Studies and experiments performed by the inventor have shown that a large portion of these differences can be attributed to the environment in which the measurement is taking place. In other words, the pressure and the temperature in the casing during measurement and, more particularly, the nature of the fluid in situ, have an effect on the obtained measurements. Indeed, the range over which these parameters may vary is very wide, with pressures of 100 bar to 1400 bar, temperatures of 20 degrees Celsius to 170 degrees Celsius, and mud densities in the range of 1 to 1.8 g/cm.sup.3. Further, it has become apparent that the measurements also depend in a non-negligible manner on the geometry of the casing being analyzed and consequently on the casing diameter and thickness. Nevertheless, the tools of the prior art are not capable of correcting for effects due to variations in these "environmental" parameters.